Justification:
This species is endemic to the Mekong basin. It is known from the Tonle Sap Lake, Tonle Sap River, and the Mekong River. It is not known to occur in the upper 2,000 km of the Mekong River. The current extent of occurrence is estimated at around 4,150 km².

Historical reports indicate that the species was abundant in the early 1900s. However, in the 1970s, local fisheries began to report the disappearance of this fish. Generation length for the species is thought to be between 10 and 15 years. Current population size is unknown, but a decline of more than 80% over the last 21 years (since 1990) can be estimated from past annual catch records, qualifying the species for Critically Endangered under criterion A.

Fishing effort in the Mekong basin in general is increasing. Fishing effort specifically for this species in the Mekong River remains constant, although it may be increasing in some areas, such as in the Tonle Sap Lake. Habitat loss and degradation are also serious threats to this fish. There has been increasing siltation of the Mekong mainstream through past deforestation practices in the northern parts of the Mekong River area. The planned destruction of rapids in the stretch of the Mekong River in the northern Lao PDR, northern Thailand and southern China may also pose a serious threat to the species' spawning habitat. The loss of migratory routes through the construction of dams may also have a negative impact on fish abundance in the river.

Given the ongoing threats to the species and its habitat, the population decline rate seen over the last 21 years is not expected to diminish over the next 24 years. Therefore, the species is assessed as Critically Endangered A4bcde.

This is a Mekong endemic species (Rainboth 1996). Historically, it was distributed throughout the Mekong River basin from the coast of Viet Nam to northern Lao PDR. Past reports of the species occurring as far north as southern Yunnan Province in China (Smith 1945, Roberts and Vidthayanon 1991) remain unconfirmed. The species' migration patterns are unknown. However, based on catch information provided by Roberts (1993) and others, it is believed that this fish migrates from the deep pools of the lower Mekong, upstream into northeast Cambodia and possibly up to Lao PDR or Thailand to spawn (Hogan et al. 2001). At least one spawning site is known (northern Thailand/Lao PDR), with a further possible spawning area in northeast Cambodia (Z. Hogan, pers. comm. 2003). There may have been other (lost) spawning sites in the middle and lower reaches of the Mekong (M. Kottelat pers. comm.). Its extent of occurrence is estimated at around 4,150 km² (Z. Hogan, pers. comm. 2003).

The current population size is unknown. A rate of population decline of over 80% can be estimated from combining annual catch data over the last thirteen years in the Mekong River Basin area:

From Thailand, there were 428 fish landed between 1983 and 2009. The species is targeted during the spawning season in Thailand and Lao (for roe). In 2010, no specimens were caught as fishing was banned. There is a quota set each year (catchment wide); in 2010 the quota was zero.

In Chiang Khong (northern Thailand), the catch has declined from a peak of 69 fish in 1990 to just seven fish in 1997 (Sretthachuea 1995, Hogan 1998). In 1999, 20 fish were captured in Chiang Khong, however no fish were caught in the area in 2001 (Hogan et al. 2001) or in 2002. In Nong Khai Province (northeast Thailand) 40-50 fish were caught per year in the early 1900s. However, since that time the number of fish caught has declined. In 1967, fishermen captured 11 fish in the area (Pookaswan 1969), and by 1970, the species occurred only rarely as bycatch in the beach seine fisheries (Pholprasith and Tavarutmaneegul 1998). Today, very few individuals are reported from Nong Khai Province.

In Luang Prabang (northern Lao PDR) the catch declined from 12 fish per year to just three fish caught in 1968. No fish were caught in 1972, 1973, or 1974 (Davidson 1975) and there has been no significant catch of the species reported since that time (Hogan et al. 2001). There are no recent data available on P. gigas catches in this area, but catches here are likely to be rare (Z. Hogan pers. comm. 2011).

In the Khone Falls (southern Lao PDR), a few fish were reported by fishermen each year prior to 1993, almost all of them in the first half of the year. No fish were reported in 1993. The status of the species in the Khone Falls area has not been assessed since 1993 (Baird, pers. comm. 2003). Since 2005, there have been some catches in the Khone Falls area; around 0-2 fish are caught each year as they move upstream and possibly over the falls (Z. Hogan pers. comm. 2011).

In the Tonle Sap River (Cambodia), four fish were captured in the bagnet fishery in 1999 and eleven fish reported in 2000. Fishermen report that they catch a few individuals each year (Hogan et al. 2001, Pengbun et al. 2001). No recent data are available from this area, but it is still likely that less than 10 Giant Catfish are caught here each year (Z. Hogan pers. comm. 2011).

Anecdotal information suggests that the species was once present in the Mekong Delta (Viet Nam), but is now reported as being very rare. One fish was caught close to, but not in, Viet Nam in 2003 (Z. Hogan, pers. comm. 2003). No significant fishery for the species exists in Viet Nam (Lenormand 1996).

Overall annual catch data for the Mekong River area indicate that around ten years ago 40-50 fish were caught each year. By 2003, the figure had dropped to approximately 5-8 catches per year (Z. Hogan, pers. comm. 2003). Since 2003, efforts to gather catch data for Giant Catfish have reduced and as a result very little data is available for recent years. However annual catches are still likely to be very low. The Tonle Sap River is one of the last places where the fish is caught in appreciable numbers. Although the species has been disappearing from Lao PDR, Thailand, and Viet Nam, there is little information on population trends in Cambodia (Hogan et al. 2001). In 2001 and 2002, no specimens were caught in northern Thailand. Annual catch figures for the Tonle Sap River in Cambodia over recent years were, four in 2000, 11 in 2001 and five in 2002.

The species is one of the world's largest freshwater fish, measuring up to three meters in length and weighing in excess of 300 kg (Smith 1945, Roberts and Vidthayanon 1991). It is a migratory species. From October to December each year, the species moves out of the lower Mekong, it is believed to migrate upstream into northeastern Cambodia and possibly Lao PDR, or Thailand to spawn (Z. Hogan et al. 2001).

The fish was bred in captivity for the first time in 2001. Individuals artificially spawned from wild-caught parents have been released into the Mekong since 1985, however this practice is now thought to have stopped and fish are now only introduced into reservoirs and not into the Mekong (Z. Hogan pers. comm. 2011). The fish almost certainly spawns upstream of Chiang Khong, Thailand. Possible spawning sites include the Kok River near Chiang Saen, Thailand, although this site requires confirmation (C. Vidthayanon pers. comm. 2011). Previously known spawning sites in the Mekong River are between Loei and Nong Khai Provinces, and in Ubon Ratchathani Province before the river fully enters Lao

First maturation is 17 years, from artificial breeding recorded of the first offspring from wild spawners in the Thai Department of Fishery's ponds. Generation length for captive fish is possibly 35 years, but this is probably not representative of the wild fish. For wild individuals, generation length has been reported as less than ten years, however this is difficult to verify. The best estimate of generation length is between 10 and 15 years (Z. Hogan pers. comm. 2003), but this is a very uncertain estimate and further research on the life history of this species is needed to confirm this.

The species historically was consumed as a high value food fish, now the species is officially protected and intentional harvesting of the species is banned. Some captive bred individuals (second generation individuals have recently been bred in ponds) have been released to supply some trade. Some wild individuals are still occasionally caught as bycatch. Captive bred stocks are frequently used in game fishing ponds in Thailand.

The major threat to the species is overfishing. The major future threat to this fish is the damming of the main stream Mekong River. Proposed dams that could impact the species if they are built include the Pak Lay, Pak Beng, Sayaboury, Luang Prabang, Latsua and Don Sahong in Laos, and the Stung Treng and Sambor in Cambodia. It is believed that sedimentation is not a threat the species, as the areas where it spawns have relatively strong currents and would stop excessive sediments from settling (W. Rainboth pers. comm.)

Alongside overfishing, main threats to the species include habitat loss and degradation (for example, as a result of damming of the Mun River and clearance of flooded forest in the Tonle Sap Great Lake), and genetic introgression with cultured stocks.

This species has been listed on CITES Appendix I since 1975. The species occurs in a Biosphere Reserve in the Tonle Sap Lake, and a Ramsar site in northeastern Cambodia, although neither of these sites offers real protection for the species. In Cambodia, it is illegal to capture, sell, or transport the species, although bagnet fisheries in the area still catch and sell the species. In Thailand, fishing for this species is regulated based on a quota license of less than 20 catches annually (C. Vidthayanon pers. comm. 2010). The species is also protected in Laos (M. Kottelat pers. comm. 2003) although this does not prevent the species being fished there.

The Thai Department of Fisheries began releasing captive-bred individuals in 1985. Between 2000 and 2003, approximately 10,000 captive-bred fish were released into the Mekong. Captive-bred individuals are no longer released into the Mekong, however they are released into reservoirs in Thailand. Large fish are now caught regularly in some Thai reservoirs but there is no evidence of self-sustaining populations. The fish have also been artificial hybridized with P. hypophthalmus for aquaculture purposes.